My son asked me this question and checking the web revealed some rules but none of them handle all sorts of exceptions.
How do I teach my son this usage ?

Deadrat earlier answered "Encourage your son to read voraciously".
Agreed, this is true and comes through experience, but now that I committed to him that I will give him some answer, I ended up trying this forum to get an answer that is explainable to a 6 year old kid.

@deadrat I'm not sure that voracity (hunger) is something which can be encouraged or induced. If you want a child to read widely, perhaps it is better to induce an interest in things.
– WS2Dec 22 '16 at 9:21

I have always found writing to be more helpful than reading. See my answer below.
– MickDec 22 '16 at 9:21

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@Rathony's suggested duplicate isn't quite a perfect duplicate, but the accepted answer on that is about as good as you're going to get, other than a relatively useless answer of "There is no set of rules in English that has no exceptions". [If someone tries to come up with an exception to that rule... they'd only be proving my point!]
– AndyTDec 22 '16 at 11:03

1 Answer
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There is no generally accepted "rule". If there was, there would be too many exceptions. As for the old "i before e, except after c" rule, which today is excoriated, I will say this: it always helps me out when I cannot remember the spellings of words that I commonly use. The only exception is "weird".

One way to learn awkward spellings that I have found helpful is the method that my old English master taught me: when you can't remember the spelling of a word, look it up and write it out (in longhand) ten or twenty times. It will soon become habit (through muscle memory), and you won't need to think about it. However, this doesn't work so well with keyboards (I think), but it served me well enough in my youth. Today, we must rely on spell-checkers.

Before I had access to automatic spell-checkers, when I was typing and could not recall the spelling of a word, I would simply write it on a piece of paper. That would usually be sufficient to jog my memory, such is the power of muscle memory. Without my writing practice, that would not have been possible.